Basically, Matus said it's due less to an anemic economy than shifting (especially aging) demographics: there are just more old people still working in America these days.

Now, as noted by Bill McBride, Merrill Lynch's Michelle Meyer has weighed in with her own take, and she basically agrees:

The two primary secular trends are the decline in the LFPR among the youth population and the rise among 55+. The LFPR for 16 to 19 year olds plunged to 34.3% last year from 52% in 2000. While this may have been accelerated by the past two recessions, we believe this is a permanent trend. On the other end, the LFPR of the older population has increased, likely reflecting higher life expectancy, less confidence in social benefit programs and loss of wealth from the Great Recession.

Meyer goes one step further and offers a projection of where the rate will go from here.

Her answer: sideways, thanks to the cyclically improving economy. Here's her chart: